Transcarboxylase is a complex enzyme made up of three different types of subunits, a central hexameric subunit (mol. wt. 360,000), 6 peripheral metallo-dimeric subunits (each of mol. wt. 120,000) and 12 biotinyl subunits (each of mol. wt. 12,000). The biotinyl subunits (2 per peripheral subunit) link 3 peripheral subunits of the two opposite faces of the central subunit. The purpose of the studies is to investigate the subunit-subunit interactions of this complex enzyme. A 42 residue non-biotinyl portion of the biotinyl subunit has been shown to be involved in binding of the peripheral subunits to the central subunit and another containing the biotin is involved in binding near the substrate sites so that the biotin can serve as a carbosyl carrier between the substrate sites. Chemical synthesis of portions of the respective amino acid sequences is underway in an endeavor to determine the requirements for these functions. The synthesized labeled peptides will be cross linked with the subunits and the complementary sequence of that subunit will be investigated. In addition, photoaffinity labeling of the CoA ester sites is under study with the purpose of determining the amino acid sequence at this site and whether more than one type site exists and the number of such sites.